Japanese Patent Laid-Open No. 235649/1988, issued Sept. 30, 1988, discloses an apparatus which controls the rotational speed of a Stirling engine. In this apparatus, the dynamo driven by the Stirling engine is connected with a main load and also with an auxiliary load via distributor. The alternating current generated by the dynamo is distributed to these loads. When the rotational speed of the Stirling engine varies, the distributor appropriately changes the ratio of the alternating current fed to the main load to the alternating current fed to the auxiliary load to let the dump electric power escape to the auxiliary load, thus maintaining the electric power supplied to the main load constant. The auxiliary load takes the form of a heater that heats up a tank in which water is stored.
During the operation of the Stirling engine, cooling water exchanges heat with a hot working medium in a cooler. Therefore, a large amount of hot water of a given temperature is obtained at all times. Furthermore, the hot water is used in various applications. On the other hand, the water heated by the heater, or the auxiliary load, is practically useless. That is, the dump power is merely discarded by heating water. This is by no means favorable from a point of view of energy saving.